I think this is because the periscope uses mirrors to ...
Nah, the top mounted periscope is an electronic imaging device (not an optical imaging device with mirrors) that can take vision enhancements and have a wireless bonus.
It is probably just a balance thing. A periscope take up a top mount and it does not provide accuracy, compensate for wind, let you get both Take Aim bonuses and does not provide a positive dice pool modifier. It is purpose built to fire around corners.
It is not really the smartgun and smartlink that provide you with the ability to fire around corners. It is the smartgun on-board
camera. You can fire around corners with any weapon you install a camera on. Does not have to be a smartgun.